1. Field of the Invention
The present invention relates to a fuel control system for internal combustion engines.
2. Description of the Prior Art
The control of fuel injection in an engine has heretofore been effected by the injectors which are opened in synchronism with the rotation of the engine and whose valve opening time T.sub.i is controlled in accordance with the input parameters. The known control methods of the type based on the input parameters may be divided into the following three broad classes. Note that there are many instances in which the actual control is performed by the use of a microcomputer. The microcomputer is provided with a microprogram and a variety of processes such as inputting operations, computational operations and outputting operations in accordance with the microprogram. Also, the principal methods which have been used for controlling the valve opening time T.sub.i are, as follows: (i) the method employing the manifold pressure and the engine speed as the principal parameters, (ii) the method employing the intake air flow and the engine speed as the principal parameters, and (iii) the method employing the throttle opening and the engine speed as the principal parameters.
Further, included among known methods improved on the basis of these three methods is one which adds the angle of peak cylinder pressure to the method of (ii) employing the intake air flow and the engine speed as the principal parameters.
The known methods of this type utilizing the detection of the angle of peak cylinder pressure are disclosed in Japanese Patent Laid-open (JP-A) Nos. 61-16269, 61-16271 and 61-16266.
These known methods are each used in accordance with its intended application. In particular, the control method mentioned in (iii) makes it prerequisite to detect the throttle opening and thus it has the advantages of being high in response and excellent in transient characteristic. However, there is a disadvantage that at low engine speeds, the engine comes into the full load operation with the reduced throttle opening and therefore the resulting accuracy is decreased. At this time, there exists a given relation between the engine speed and the throttle opening.
On the other hand, as regards the requirements of the engine, there is a desire for a lean combustion operation. The methods designed to meet this requirement are referred to as the lean combustion type. The lean combustion type has the effect of improving the fuel consumption and improving the exhaust emission control performance.
The fuel injection method must be low in cost and high in performance. While the control method of the above (iii) is excellent for this purpose, it has the previously mentioned deficiencies.
Also, where the lean combustion operation is effected, there exists a stable combustion limit and exceeding this limit causes the rotation of the engine to become unstable.